1. Field of the Invention
The present invention relates to a technique for adding watermarked information to an image such as a document image.
2. Description of the Related Art
If characters or graphics are printed on a printing medium such as a copy paper and the printed matter is output, it is possible to easily grasp contents printed on the printed matter by seeing the text or image printed on the printed matter.
On the other hand, there exists “digital watermarking” in which information for preventing copying or counterfeiting or secret information is embedded in the digital data such as graphic data and text data. There exists a technique for preventing dishonesty such as a counterfeit by embedding the “digital watermarking” in the text data or the like.
In recent years, the “digital watermarking” is included in a document image by disposing and rendering a black picture element (or black dot) in a background of the document image using a predetermined method (e.g., see Japanese Patent No. 3628312).
As shown in FIG. 19, if a watermarked image 103 is embedded in a background of a document image 102 by a watermarked information embedding apparatus, a document image 104 in which the watermarked image 103 is embedded can be printed.
As shown in the watermarked document image 104, a tint block of the watermarked image 103 is superposed on “character”, and a portion of the tint block disappears. However, if the watermarked information is provided with redundancy, it is possible to precisely read the watermarked information.
However, when the background of the document image is shaded, since the shaded figure and the tint block of the watermarked information are superposed on each other, this exceeds the redundancy of the watermarked information, and it is difficult to precisely read the watermarked information.
As shown in FIG. 20, it can be found that a shaded image 112 exists in a document image 111 together with characters. As shown in the watermarked document image 114, since the figure of the shaded image 112 and the tint block of the watermarked information 113 are superposed on each other, it is difficult to precisely read the watermarked information in this superposed portion.
When the document image 111 is shaded over its wide range like the shaded image 112 existing in the document image 111, even if the watermarked information 113 is provided with sufficient redundancy, it is not possible to precisely read the watermarked information in the shaded portion. Ditto for a photograph image and an illustration image other tan the shaded image 112.